We propose to study the degree of completeness of functional development of autonomic innervation of the canine heart at birth and relate any subsequent maturational changes to age. We also plan to examine if asymmetrical developmental cardiac sympathetic nerve (CSN) changes play a significant role in altering the cardiac electrophysiologic state and especially the arrhythmiogenesis in early life. By measuring refractory period (RP) changes during stimulation of the CSN in puppies 1 to 6 weeks of age, we found that the functional sympathetic innervation of the heart is incomplete at birth. A peak in individual nerve activity at the second week, with asymmetrical decline at the third week, and then a gradual increase and leveling off in subsequent weeks was identified. This asymmetrical sympathetic development may form the basis for sympathetic imbalance in early life. Also by measuring ventricular fibrillation threshold (VFT) changes during stimulation of the CSNs in puppies 1 to 6 weeks, we found that vulnerability to fibrillation the first week was minimal but progressively increased through the sixth. We also found that stellate ganglia stimulation had an opposite effect on VFT changes than that of the distal CSNs, mainly the ventrolateral. The ventrolateral was the only nerve that decreased VFT linearly during the first six weeks of life. In addition we plan to examine the functional development of parasympathetic innervation of the heart and assess in electrophysiological terms the degree and importance of sympathetic-parasympathetic interaction in early life.